Peg, youngster endlessly (3)
I believe the answer is:
tee
'peg' is the definition.
(tee is a kind of peg)
'youngster endlessly' is the wordplay.
'youngster' becomes 'teen' (I've seen this before).
'endlessly' means to remove the last letter.
'teen' with its last letter taken off is 'TEE'.
(Other definitions for tee that I've seen before include "Drive off it, half-tittering" , "Golf starting point" , "support of leisure activity" , "Golfer's peg" , "Start of a hole in golf" .)